1. Field of the Invention
The present invention relates to a semiconductor device, and, more particularly, to a so-called die bonding material which is used to secure a semiconductor chip such as an IC chip to an island portion, that is the portion of a lead frame on which the semiconductor chip is mounted.
2. Description of the Related Art
At present, resin die bonding materials which are formed by adding a silver, silica, or alumina filler and the like to an epoxy resin or a polyimide resin for the purpose of improving the material's properties with respect to electrical conductance, heat transfer characteristics, mechanical characteristics and the like are used primarily for die bonding materials.
With respect to such a resin die bonding material, however, (1) it is necessary to cure the material at a high temperature of 150.degree. to 400.degree. C. In addition, (2) in a wire bonding process subsequent to die bonding, this material is further subjected to wire bonding at a high temperature of 250.degree. to 350.degree. C.
Consequently, since the aforementioned processes, particularly the process (2), involve high-temperature processing, the resin die bonding materials undergo heat deterioration due to the high temperature, and the resulting decomposed gas and the like adversely affect the reliability of the semiconductor device. Accordingly, the resin die bonding materials must excel in heat resistance so that such adverse effects can be eliminated.
In the case of a die bonding material using a polyimide resin, no particular problem exists in this respect since its molecular structure is rigid and it excels in heat resistance. In the case of an epoxy resin, however, it is necessary to cope with this problem by using one whose glass transition temperature Tg is set to at least 100.degree. C. or above. Accordingly, a die bonding material using either of these resins is hard at room temperature, and this hard state of the resins continues up to a temperature range near the glass transition temperature Tg of the materials. Particularly in the case of the polyimide resin, the hard state continues up to a high temperature of about 300.degree.-400.degree. C.
With such a material, when the lead frame to which the chip is secured is cooled down to room temperature after die bonding or wire bonding at a high temperature, there are cases where the chip warps into a convex shape due to a bimetal effect. The occurrence of this warping phenomenon is attributable to the fact that, since the coefficient of thermal expansion of the lead frame is greater than that of the chip, in the case of a conventional hard die bonding material, the heat shrinkage of the lead frame is directly transmitted to the chip. This phenomenon occurs to a noticeable extent particularly when the lead frame is formed of a copper-based material.
If the chip thus undergoes deformation, fine cracks occur in a passivating film or the like present on the chip surface, presenting great problems in terms of the reliability of the semiconductor device. Particularly when the size of the chip is large and warping is extensive, breakage of the chip occurs, with the result that it becomes impossible to use the semiconductor device.